1. Field of the invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus for forming an image on a sheet in an electrophotographic method which has steps of charge, exposure, development and transfer.
2. Description of Related Art
An electrophotographic method has, in a relationship between an original image and a reproduced image, a normal development mode wherein a positive image is obtained from a positive original and a reverse development mode wherein a positive image is obtained from a negative original. In a reader printer which exposes a microfilm to a light and projects its image on a screen or reproduces an image on a sheet in an electrophotographic method, since the microfilm can be positive and negative, the reader printer should be able to carry out the normal developing mode and the reverse developing mode selectively.
Thus, on the premise that one kind of toner which is charged with a specified polarity is used in a development process, a reader printer which has a bipolar photosensitive member is provided. The bipolar photosensitive member can be charged to have either a positive polarity or a negative polarity. For example, when the microfilm is positive, the bipolar photosensitive member is charged to have the negative polarity to form a positive latent image. A toner which is charged to a positive polarity is deposited on an image portion which has a high electric potential. On the other hand, when the microfilm is negative, the bipolar photosensitive member is charged to have positive polarity to form a negative latent image. In this case, the toner which is charged to have the positive polarity is deposited on the formed latent image which has a low electric potential.
However, right after the charge polarity is switched, the conventional bipolar photosensitive member can not be charged with a specified voltage by a charger set at a specified output value. Therefore, inconveniences such as lowering of the toner depositing amount (lowering of the image density) in the normal development mode and toner deposition on the background in the reverse development mode occur. Thus, right after the image forming mode is switched, the bipolar photosensitive member is rotated by a specified amount to be charged for elimination of the influence of the previous charging (this is called "pre-charging" following description). However, for purposes of rapidity, it is preferable that the pre-charging process is eliminated or finished in a short time.